Bleaching using oxygen is already known, with the most important bleaching agents being hydrogen peroxide and sodium peroxide. Sodium perborate (NaBO.sub.3 .multidot.4 H.sub.2 O) is also of great importance, but has not been used in industrial bleaching for economic reasons; however, it has been used as a component of automatic detergents.
Peroxyacetic acid has posed handling difficulties in previous processes and has been used to a very limited extent for bleaching naphthol-dyed textiles. Potassium persulfate has also been proposed for bleaching as has sodium persulfate, as an additive in peroxide bleaching.
Molecular oxygen has likewise been used successfully as a bleaching agent in the MODO and SAPPI processes, with oxygen treatment taking place in the presence of NaOH at a temperature of approximately 100.degree. C. To suppress over oxidation of the cellulose, MgCO.sub.3 or complex manganese compounds are used.
Known bleaching processes using oxygen and peroxo compounds require temperatures of at least 100.degree. C. and a generally highly alkaline medium generally produced by using alkali hydroxides.
Activation and stabilization are critical for the bleaching process.
The purpose of stabilization is to prevent radical decomposition of hydrogen peroxide and to render inactive any catalyst present which may promote this breakdown.
One stabilizer commonly employed in peroxide bleaching is water glass, often used in combination with magnesium salts. Other stabilizers include magnesium salts together with dispersing agents or complex formers. Other known stabilizers for hydrogen peroxide are phosphorus salts such as Na.sub.2 H.sub.2 P.sub.2 O.sub.7.
Activation is effected by raising the temperature, alkalinity, and concentration. Activation begins and accelerates the bleaching process, but also favors the occurrence of undersirable side reactions, especially the breakdown of cellulose.
Disproportionation of peroxo compounds proceeds for H.sub.2 O.sub.2 for example according to the following general reaction formula: ##STR1##
Two O atoms have increased their oxidation number by +1; two O atoms have reduced their oxidation number by -1.
The same is true for the disproportionation of peroxo acids, for example peroxoacetic acid. ##STR2##
If disproportionation of the peroxo compounds does not take place through radicals, molecular oxygen is then released in a form which initiates oxidation reactions at low temperatures. As a result, oxygen form disproportionation which does not proceed through radicals permits a very protective form of oxidation, avoiding side reactions.
A chlorine-free multistage cellulose bleaching process is known form DE-OS 22 19 505, with at least three bleaching stages being used, with bleaching conducted using a peroxide in the first stage, a peracid in the second stage, and again, a peroxide in the third stage. This process uses neither chlorine nor water glass. NaOH is preferably used as the base, with the bleaching temperature being 20.degree. to 140.degree. C., preferably between 40.degree. and the boiling point of the bleaching bath.
Under these conditions, decomposition to oxygen radicals also takes place, which then initiates largely uncontrolled oxidation by the oxygen radicals.
Bleaching which is completed in a short time with oxygen from disproportionation that does not proceed through radicals does not take place here. Bleaching times for each stage are four hours at about 70.degree. C. and about 20 minutes at 100.degree. C.